In general, the present invention is widely applicable to the production of materials for electronic device such as semiconductors or semiconductor devices, and liquid crystal devices. For the convenience of explanation, however, the background art relating to semiconductor devices as an example of the electronic devices, will be described here.
Substrates for semiconductors or electronic device materials such as silicon have been subjected to various kinds of treatments such as formation of an insulating film such as oxide film, film formation by CVD (chemical vapor deposition), etc., and etching.
It is not too much to say that the development in the performances of semiconductor devices in recent years is attributable to the microfabrication technique concerning the semiconductor devices such transistor. At present, the microfabrication technique concerning the semiconductor devices is being improved for the purpose of attaining further development in the performances of semiconductor devices. According to the recent requirement for forming microstructures and attaining further development in the performances in the field of semiconductor devices, the demand for an insulating film having a higher performance (for example, in view of leakage current) has been increased remarkably. This is because the leakage current of a certain degree can cause a severe problem in the recent devices which have attained finer structure, and/or higher performances, even when the leakage current of such a degree have actually caused substantially no problem in the conventional devices having a lower degree of integration. Particularly, in view of the development in the mobile or portable-type electronic devices in a so-called “ubiquitous” society of recent construction (i.e., information-oriented society wherein people can use a network service, anytime and anywhere, by means of electronic devices), the reduction in the leakage current is an extremely important issue.
For example, with respect to the development of a next-generation MOS transistor, as the above-mentioned microfabrication technique is advanced, the film-thinning of a gate insulator have approached a limit thereof, and a serious problem to be overcome is brought into view. More specifically, in view of processing technique, it is possible to thin the film thickness of a silicon oxide (Sio2) film which has been used as a gate insulator, to the utmost limit thereof (i.e., a level corresponding to one or two atom-layer). However, when film thickness is reduced to 2 nm or less, an exponential increase in the leakage current is caused by the direct tunneling due to quantum effect, whereby the resultant power consumption is problematically increased.
At present, the IT (information technology) market is going to be transformed from the stationary-type electronic devices represented by desktop type personal computers or home telephones (i.e., devices supplied with electricity from a plug socket) into “ubiquitous network society” wherein people can access the Internet anywhere and anytime. Accordingly, it is considered that mobile terminals such as cellular phone or car navigation system will become predominant in the near future. Such mobile terminals, per se is required to be a high-performance device. In addition, they should satisfy a prerequisite that they are small-sized, light in weight, and have a function capable of being used for a long time, although these performances are not necessarily required for the stationary-type devices. Accordingly, in the field of a mobile terminal, it is an extremely important issue to accomplish the reduction in power consumption and to accomplish the above-mentioned high performances simultaneously.
Typically, for example, with respect to the development of the above-mentioned next-generation MOS transistor, when the microfabrication of a high-performance silicon LSI is investigated, there occurs a problem that the leakage current is increased and the resultant power consumption is also increased. Accordingly, in order to accomplish a higher performance while reducing the power consumption, it is necessary that the performance of an MOS transistor is enhanced without increasing the gate leakage current therein.
With respect to the above-mentioned microfabrication, due to the development in the microfabrication technique, at present, it is nearly possible to produce a super-microfabricated semiconductor device (such as MOS transistor) having a gate length of 0.1 μm or less.
In such a super-microfabricated semiconductor device when the working speed of a semiconductor device is intended to be increased along with the shortening of the gate length, it is necessary to reduce the thickness of gate insulator in accordance with the scaling law. For example, when a conventional thermal oxidation film is used as the gate insulator, it is necessary to reduce the thickness of the gate insulator to about 1.7 nm or less, for example. However, when the thickness of the conventional oxide film is reduced in this way, the gate leakage current flowing through the oxide film is increased due to the above-mentioned tunnel effect.
For the above reason, heretofore, it has been investigated to use a high-dielectric film such as Ta2O5 or ZrO2 as the gate insulator, instead of the conventional silicon oxide film. However, various properties of the high-dielectric film such as Ta2O5 or ZrO2 are quite different from those of the silicon oxide film which has heretofore been used in the semiconductor technology. Accordingly, there remain a lot of problems to be solved, before such a high-dielectric film can actually be used as the gate insulator.
As a measure for solving such problems, it has been investigated to use a nitride film (and/or oxynitride film) as the gate insulator material. For example, the silicon nitride is the material which has been used in the conventional semiconductor processes. In addition, the silicon nitride has a relative dielectric constant which is about twice that the silicon oxide film, and is promising as the gate insulator for of the next-generation high-speed semiconductor devices.
Heretofore, it has been usual that the silicon nitride film is formed on an interlayer dielectric (or interlayer insulating film) by using a plasma-CVD method. However, such a CVD nitride film generally provides a large leakage current and also has a large absolute value of vfb (flat band voltage), and therefore it is not suitable for the gate insulator. For this reason, it has never been attempted to use the nitride film as the insulating film constituting a gate electrode.
On the other hand, there has recently been proposed a technique that a nitrogen-containing gas such as nitrogen gas, nitrogen gas and hydrogen gas, or NH3 gas is introduced into a microwave-excited inert gas plasma such as argon or krypton, so as to generate an nitrogen radicals or NH radicals (however, the NH radicals are liable to provide dangling bonds), whereby the surface of a silicon oxide film is converted into a nitride film. The thus formed nitride film may provide a leakage current characteristic which is comparable to, or even superior to that of a thermal oxidation film, and is promising as the gate insulator of the next-generation high-speed semiconductor devices. In addition, there has been proposed a technique that the surface of a substrate for electronic device is directly nitrided by such microwave plasma.
However, in the prior art, e.g., when the surface of a silicon oxide film formed on a substrate for semiconductor is modified or transformed by microwave-excited hydrogen nitride radicals NH*, there occurs a degradation in the resultant electric properties (for example, an increase in the absolute value of Vfb, a change in threshold voltage), and a desired transistor characteristic has never been not accomplished.